There is an increasing demand for many plastic products to be biodegradable.
Biodegradable polymers based on starch are known and inexpensive but generally possess high viscosities and poor melt properties that make them consequently difficult to process. Products made from starch are often brittle and water sensitive. Difficulties have been encountered in producing starch based polymers particularly for injection moulding. The molecular structure of the starch is adversely affected by the shear stresses and temperature conditions needed to plasticise the starch and pass it through an extrusion die. For most products foaming has to be avoided and this generally requires attention because of the water content of the starch. Foaming has been avoided by degassing the melt prior to exiting the die as suggested in U.S. Pat. Nos. 5,314,754 and 5,316,578. The latter patent also avoids adding water to the starch. As explained in U.S. Pat. No. 5,569,692 by not drying starch and avoiding the addition of water the starch can be processed at temperatures between 120° C. and 170° C. because the water bound to the starch does not generate a vapour pressure such as to require high pressures.
Another approach to improving the melt processability of starch is to provide an additive as in U.S. Pat. No. 5,362,777 which reduces the melting point of the starch. The additive is selected from dimethyl sulfoxide, a selection of polyols and amino or amide compounds.
U.S. Pat. No. 5,043,196 discloses a high amylose starch for injection moulding.
U.S. Pat. No. 5,162,392 discloses an injection mouldable corn starch and LDPE biodegradable polymer.
In order to produce starch polymers for particular applications they have been blended with a range of other polymers. Biodegradable blown films are disclosed in U.S. Pat. No. 5,322,866 which blends raw starch, polyvinyl alcohol and talc with glycerol and water. U.S. Pat. No. 5,449,708 discloses compositions of starch ethylene acrylic acid and a salt of stearic acid plus a glycerol based lubricant. Flexible and clear transparent sheets are disclosed in U.S. Pat. No. 5,374,304. These are composed of a high amylose starch and a glycerol plasticizer. The use of starch in conjunction with high amylose or modified starches has also been proposed. U.S. Pat. Nos. 5,314,754 and 5,316,578 both suggest the use of modified starches including hydroxypropyl substituted starches. Hydroxypropylation reportedly increases elongation at break and burst strength and improved resilience in the polymers.
International application WO00/36006 discloses biodegradable water soluble formulations using a major amount of modified starch and a minor amount of a water soluble poly vinyl alcohol. These formulations are thermo formable but there are no examples of injection mouldable compositions.
The foregoing discussion of the prior art shall not be understood to be equivalent to an acknowledgement of the general state of knowledge in the field of biodegradable injection mouldable polymer compositions.
It is an object of this invention to provide injection mouldable biodegradable polymer compositions which can be readily processed at higher melt processing temperatures for good flow but which have low moisture content so as to avoid bubbling and discolouration during processing and acceptable properties for their intended use.